High-coherence semiconductor light sources

ABSTRACT

A laser resonator includes an active material, which amplifies light associated with an optical gain of the resonator, and passive materials disposed in proximity with the active material. The resonator oscillates over one or more optical modes, each of which corresponds to a particular spatial energy distribution and resonant frequency. Based on a characteristic of the passive materials, for the particular spatial energy distribution corresponding to at least one of the optical modes, a preponderant portion of optical energy is distributed apart from the active material. The passive materials may include a low loss material, which stores the preponderant optical energy portion distributed apart from the active material, and a buffer material disposed between the low loss material and the active material, which controls a ratio of the optical energy stored in the low loss material to a portion of the optical energy in the active material. A Vernier grating and tuning mechanism can be used to tune the low-noise laser.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 62/113,267 filed on Feb. 6, 2015, the disclosure of which isincorporated by reference in its entirety. The present application isalso a continuation-in-part application of U.S. patent application Ser.No. 14/319,466 filed on Jun. 30, 2014, which, in turn, claims priorityto U.S. Provisional Application No. 61/842,770 filed on Jul. 3, 2013,the disclosures of both of which are incorporated herein by reference intheir entirety.

STATEMENT OF GOVERNMENT GRANT

The present Application relates to an invention that was made withsupport of the United States government under contract no.W911NF-10-1-0103 awarded by the U.S. Army Research Office and undercontract no. HR0011-12-C-0006 awarded by the Defense Advanced ResearchProjects Agency (DARPA) of the U.S. Department of Defense. The U.S.government has certain rights in an invention related to the presentApplication.

STATEMENT OF JOINT RESEARCH AGREEMENT

The present Application was developed and the claimed invention was madeby, or on behalf of, one or more parties to a joint research agreement.The parties in the joint research agreement are California Institute ofTechnology and Telaris Inc.

TECHNICAL FIELD

Embodiments of the present invention relate generally to light sources.More particularly, example embodiments of the present invention relatesto high-coherence semiconductor light sources.

BACKGROUND

A distributed feedback (DFB) laser may be disposed within asemiconductor device such as a laser diode (LD). An active region of theDFB LD may comprise a diffraction grating to provide optical feedbackfor the laser over a narrow wavelength band, which may be selectedaccording to the pitch with which the grating is fabricated. The narrowoutput spectrum characteristic of DFB lasers gave rise to theirusefulness in optical communications, in which information is exchangedover networks of optical fiber and other transmission media.Conventional semiconductor distributed feedback lasers have been used aslight sources for powering the fiber-optic based internet and relatednetworks since the mid-1970s. Since then, volumes of network traffichave exploded, along with concomitant demand for higher bandwidth andincreased data rates.

For example, the explosive growth of the internet over the last 20 yearshas created a geometrically increasing demand for bandwidth. Existingcommunication approaches typically meet this bandwidth demand by anoptical fiber network with multiple channels. Using dense wavelengthdivision multiplexing (DWDM) techniques, each of the multiple channelscomprises an optical wavelength different from an optical wavelength ofeach of the other channels.

Information is transmitted over DWDM networks at 1 bit per pulse bymodulating the intensity of the light source (e.g., on/off keying) atspeeds up to 10 Gb/s. The upper data rate is typically limited byoptical impairments, which are induced or introduced by the opticalfiber transmission media. The full utilization of the available numberof channels in the optical spectrum along with the bound on modulationrates has instigated the search for alternative information transmissionschemes to meet the ever increasing bandwidth demand.

The growing demand for transmitting information at ever-higher datarates has led to the development of coherent communication, in whichinformation is encoded on an optical wave using principally a modulationof its phase. Quantum-based limitations related to their inherent phaseor temporal coherence characteristics limit the phase stability ofconventional DFB lasers. The channel capacity of conventional DFB lasersis thus insufficient for handling the demands imposed by the migrationof networks to coherent communication.

Improved coherence has thus been long sought in semiconductor DFBlasers. Previous approaches have used elongation of the laser cavities,multiple phase-shifts for the engineering of longitudinal modes therein,optimization of the active laser medium, e.g. strained QW (quantumwell), and wavelength detuning.

The spectral linewidths achieved using such techniques in commercial andother conventional or state of the art lasers however remainpersistently high. For example, spectral linewidths of conventional DFBlasers remains above 100 kHz, and this linewidth value itself reflects anarrowness that may be attained only using high pump currents. Moreover,linewidths at this level remain too high to satisfy the demandspresented by multi-phase coherent communication and other usefulapplications.

Some fiber based lasers, which have linewidths below 1 kHz, and externalcavity lasers (ECL), which have linewidths below 10 kHz, have highcoherence characteristics. However, they typically have bulky andcomplex structures, which render them incompatible with the physicalscaling demands of growing networks.

Contemporary optical communication networks are powered extensively bysemiconductor lasers, including conventional DFB LDs, because of thebenefits of their small size, high power output, high efficiency, lowcost, and potential integration opportunities with associated electroniccircuits. Due, however, to their significant phase noisecharacteristics, primarily of intrinsic quantum mechanical origin,conventional semiconductor lasers, including typical conventional DFBLDs, are typically incapable of meeting the stringent spectral purityrequirements to ultra-high-speed communication networks.

A semiconductor distributed feedback laser capable of handling therequirements imposed by coherent communication networks, without usingDWDM would thus be useful. A distributed feedback laser free of thequantum-based phase or temporal coherence characteristics inherent inconventional DFB lasers would also thus be useful. It would further beuseful to improve significantly the phase or temporal coherencecharacteristics and the channel capacity of a distributed feedback laserrelative to conventional DFB LDs.

Approaches described in this section may, but not necessarily, have beenconceived or pursued previously. Unless otherwise indicated, it shouldnot be assumed that any approaches discussed above include any allegedprior art merely by any such discussion. Not dissimilarly, any issuesdiscussed in relation to any of these approaches should not be assumedto have been recognized in any alleged prior art merely based on anysuch discussion above.

SUMMARY

Example embodiments of the present invention relate to a resonator for alaser device. The laser resonator has at least one active material foramplifying light associated with an optical gain of the resonator. Thelaser resonator also has one or more passive materials disposed inproximity with the at least one active material wherein the resonatoroscillates over one or more optical modes, each of the one or moreoptical modes corresponding to a particular spatial energy distributionand resonant frequency, and wherein, based on a characteristic of theone or more passive materials, for the particular spatial energydistribution corresponding to at least one of the one or more opticalmodes, a preponderant portion of optical energy is distributed apartfrom the active material.

The one or more passive materials may include a low loss material forstoring optical energy of the preponderant portion distributed apartfrom the active material. The one or more passive materials may alsoinclude a buffer material disposed between the low loss material and theat least one active material for controlling a ratio of the opticalenergy stored in the low loss material to a portion of the opticalenergy in the active material. The buffer material may include amaterial like silicon dioxide, which has a low refractive index. Theactive material may include a III-V material and the low loss materialmay include silicon. The passive materials may be disposed in layers, atleast one of which is bonded (e.g., with wafer bonding) with a layerwith active material.

The low loss passive material may be configured with a pattern of holes.The configured hole pattern may determine an oscillation frequency, anoutput rate and an output mode profile of the resonator and detersspontaneous emission therefrom. The pattern of holes may have aone-dimensional (1D) configuration, such as a linear or near linearaspect. The configured pattern of holes may include an approximatelyuniform array of holes of approximately uniform size, and a defect(e.g., related to the approximate uniform size) disposed within (e.g.,over, in or nearly in a center area of) the approximately uniform arrayof holes.

Example embodiments of the present invention also relate to laserdevices with such resonators disposed on semiconductor dies. The laserdevices may have a heat sink component attached to the semiconductor dieand configured therewith for removing heat generated in the activematerial from the resonator of the laser device. The heat sink may beattached to the semiconductor die with an epitaxial-side-downconfiguration in relation to the active material of the laser resonator(or in other configurations). The laser device may also have a detectorcomponent attached to the semiconductor die and configured therewith fordetermining an output characteristic of the laser resonator. The outputcharacteristic may relate to measuring a frequency noise relatedcomponent of the resonator output, which may include computing a highfrequency noise spectrum of the resonator output and suppressingmeasurement of low frequency fluctuations of the resonator output.

In some embodiments, the present disclosure describes a laser resonatorcomprising: at least one active material providing optical gain foramplifying light within the resonator; one or more passive materialsdisposed in proximity with the at least one active material, wherein theone or more passive materials comprise: a low loss material layerconfigured to store optical energy of a preponderant portion distributedapart from the active material, a mode control layer disposed betweenthe at least one active material and the low loss material layer,wherein: the low loss material layer or the mode control layer or boththe low loss material layer and the mode control layer comprise tunablegratings, the mode control layer is configured to control a ratio of theoptical energy stored in the low loss material to a portion of theoptical energy in the active material the laser resonator deviceoscillates over one or more optical modes, each of the one or moreoptical modes corresponding to a particular spatial energy distributionand resonant frequency, based on a characteristic of the one or morepassive materials, for the particular spatial energy distributioncorresponding to at least one of the one or more optical modes, apreponderant portion of optical energy is distributed apart from theactive material, and a thickness of the mode control layer is configuredto transversely distribute the preponderant portion of the opticalenergy apart from the active material and maintain an evanescent fieldin the active material.

Lasing refers herein to generating coherent light over one or moreinfrared, visible, ultraviolet, etc. wavelengths by a laser (lightamplification by stimulated emission of radiation) process. Exampleembodiments of the present invention relate to a method for lasing,which includes amplifying light associated with an optical gain in atleast one active material of an optical resonator, and distributingspatial energy within the resonator, in which one or more passivematerials are disposed in proximity with the at least one activematerial, in which the resonator oscillates over one or more opticalmodes, each of the one or more optical modes corresponding to aparticular spatial energy distribution and resonant frequency, and inwhich, based on a characteristic of the one or more passive materials,for the particular spatial energy distribution corresponding to at leastone of the one or more optical modes, a preponderant portion of opticalenergy is distributed apart from the active material.

The one or more passive materials may include a low loss material forstoring optical energy of the preponderant portion distributed apartfrom the active material, as well as a buffer material, which isdisposed between the low loss material and the at least one activematerial and effectively controls a ratio of the optical energy storedin the low loss material to a portion of the optical energy in theactive material. Example embodiments relate to lasers, e.g., systems,devices products, etc. for sustaining such lasing processes, as well asto methods for fabricating them.

Example embodiments may thus implement a high-Q, separated functionsemiconductor laser (high-Q SFL), which is characterized by a phasecoherence with an order of magnitude improvement over high qualityconventional DFB lasers. For example, an embodiment of the presentinvention is implemented with a high-Q SFL having a spectral linewidthof 18 Kilohertz (kHz) at the commonly used optical communicationwavelength of 1.55 micrometers (μm).

An example embodiment of the present invention is implemented usingoptical phase and/or quadrature amplitude modulation to encodeinformation over a complex 2D phase plane. The use of opticalphase/quadrature amplitude modulation (PQAM) may thus obviate DWDM.

The example implementation may use coherent detection (CD) at a receiverto recover the full field of an optical signal thus encoded at atransmitter, including the amplitude and phase of the signal. Coherentdetection allows an embodiment of the present invention to correct orcompensate for fiber-induced optical impairments, such as chromatic andpolarization mode dispersion, using digital signal processing (DSP)techniques. The example implementation may thus exceed the capability ofthe typically-used direct detection approaches.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the present invention are described below inrelation to a high-coherence semiconductor light source. The descriptionbelow refers to the following drawings, which comprise a part of thespecification of the present Application. In the drawings:

FIG. 1 depicts example constellation diagrams representing modulationschemes of various spectral efficiencies;

FIG. 2 depicts an example of noise effects in phase modulation,according to an embodiment of the present invention;

FIG. 3 depicts a phasor representation of an example spontaneousemission event, according to an embodiment of the present invention;

FIG. 4 depicts a schematic cross-section of a first example hybridSi/III-V Laser, according to an embodiment of the present invention;

FIG. 5 depicts a top view of an example high-Q photonic resonator,according to an embodiment of the present invention;

FIG. 6 depicts a schematic cross-section of a second example hybridSi/III-V Laser, according to an embodiment of the present invention,according to an embodiment of the present invention;

FIGS. 7A and 7B depict a representation of quantum noise effects onlinewidth;

FIG. 8A depicts a first example heat dissipation configuration,according to an embodiment of the present invention;

FIG. 8B depicts a second example heat dissipation configuration,according to an embodiment of the present invention;

FIG. 9 depicts a flowchart for an example fabrication process, accordingto an embodiment of the present invention;

FIG. 10 depicts a cross-section of an example High-Q SFL device,according to an embodiment of the present invention;

FIG. 11A depicts a perspective of the example High-Q SFL device,according to an embodiment of the present invention;

FIGS. 11B-11C depict an example grating component of the High-Q SFLdevice, according to an embodiment of the present invention;

FIG. 12A depicts a top view of the geometry of the example gratingcomponent, according to an embodiment of the present invention;

FIG. 12B depicts a spatial band structure of an example High-Q hybridresonator plotted against a transmission spectrum simulated in relationthereto, according to an embodiment of the present invention;

FIG. 12C depicts a dispersion diagram of an example local unit cell,according to an embodiment of the present invention;

FIG. 12D depicts an intensity profile simulated for the longitudinalfield of an example High-Q hybrid resonator, according to an embodimentof the present invention;

FIG. 12E depicts a Fourier component amplitude distribution of thelongitudinal field of an example High-Q hybrid resonator, according toan embodiment of the present invention;

FIG. 12F depicts an emission spectrum simulated for the example High-QSFL, according to an embodiment of the present invention;

FIG. 13A depicts optical power plotted against pump current over variousoperating temperatures, according to an example embodiment of thepresent invention;

FIG. 13B depicts light (optical power output) plotted against pumpcurrent over various operating temperatures, according to an exampleembodiment of the present invention;

FIG. 13C depicts optical spectra of example High-Q SFLs at a given pumpcurrent and at a certain temperature, according to an embodiment of thepresent invention;

FIG. 13D depicts optical spectra of example High-Q SFLs of variousgrating periods at a given driving current and a certain temperature,according to an embodiment of the present invention;

FIG. 14 depicts a frequency noise spectrum of an example High-Q SFL,according to an embodiment of the present invention;

FIG. 15A depicts a Schawlow-Townes linewidth of an example High-Q SFL asa function of the offset pump current from a threshold, according to anembodiment of the present invention;

FIG. 15B depicts a distribution of Schawlow-Townes linewidths as afunction of their respective emission wavelengths, for example High-QSFL devices implemented over multiple laser bars fabricated on severalseparate semiconductor chips, according to an embodiment of the presentinvention;

FIG. 16A depicts a modal energy distribution typical of conventionallasers; and,

FIG. 16B depicts an example modal energy distribution for a laser,according to an embodiment of the present invention;

FIG. 17 illustrates an extension of the quantum noise control concept toa tunable SGDBR laser; paragraphs have to be increased by one from nowon.

No scale applies in these drawings unless and except as specificallystated, as with reference to the vertical axis and the horizontal axisshown in Figure (FIG. 7A, etc.

DETAILED DESCRIPTION

Laser resonators are described herein in relation to examplehigh-coherence semiconductor light sources. Example embodiments aredescribed in relation to a high-Q separated function hybrid laser. Inthe following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however,that embodiments of the present invention may be practiced without thesespecific details. In other instances, well-known structures and devicesare not described in exhaustive detail, in order to avoid unnecessarilyobfuscating, obstructing, obscuring, or occluding aspects of embodimentsof the present invention.

Overview

A laser resonator includes an active material, which amplifies lightassociated with an optical gain of the resonator, and passive materialsdisposed in proximity with the active material. The resonator oscillatesover one or more optical modes, each of which corresponds to aparticular spatial energy distribution and resonant frequency. Based ona characteristic of the passive materials, for the particular spatialenergy distribution corresponding to at least one of the optical modes,a preponderant portion of optical energy is distributed apart from theactive material. The passive materials may include a low loss material,which stores the preponderant optical energy portion distributed apartfrom the active material, and a buffer material disposed between the lowloss material and the active material, which controls a ratio of theoptical energy stored in the low loss material to a portion of theoptical energy in the active material.

Optical Phase/Quadrature Amplitude Modulation

An example embodiment of the present invention is implemented usingoptical phase and/or quadrature amplitude modulation to encodeinformation over a complex 2D phase plane. The use of opticalphase/quadrature amplitude modulation (PQAM) may obviate (or be combinedwith) conventional DWDM approaches and allows the use of exampleembodiments in coherent communications and other applications. FIG. 1depicts example constellation diagrams representing modulation schemesof various spectral efficiencies.

Encoding information in the phase of the optical carrier as opposed toonly its amplitude, allows for increase in spectral efficiency (bits/Hz)and thus more efficient utilization of the available spectrum. Forexample, modulating an optical signal according to its intensity allowseach amplitude value, e.g., ‘on’ or ‘off’, to represent only a singledata bit e.g., ‘0’ or ‘1’, respectively.

Somewhat more spectral efficiency may be achieved using QPSK (quadraturephase shift keying) and 8-PSK modulation schema, which respectivelyallow each symbol disposed over the complex plane to represent two (2)symbols (e.g., 00, 01, 10, 11), and three (3) symbols (e.g., 000, 001,010, 000, etc.). In a 16-QAM (quadrature amplitude modulation) scheme,four (4) bits can be transmitted per pulse, which represents aquadrupling of bandwidth at a given modulation (baud) rate overconventional intensity modulation.

Commercial communication links may now operate at data rates of 100 Gb/sper channel based on dual polarization, quadrature phase shift keying(DP-QPSK), and “next-generation” lines with rates of 400 Gb/s lines arenow under development. Example embodiments of the present invention maybe implemented using higher-level modulation formats (e.g. 16, 64,256-QAM). Coupled with the use of DWDM techniques, an example embodimentmay be used to provide Tb/s-scale network bandwidths.

Raising the spectral purity of laser sources allows them to transmitinformation using the more efficient phase constellations of highercomplexity. However, spontaneous emission and other such quantum noiseeffects may scatter, spread or smear phase based signal information.Such inherent phase noise characteristics causes a spread in the phaseof the symbols transmitted therewith. FIG. 2 depicts an example of noiseeffects in phase modulation. As seen in FIG. 2, phase noise may degradethe certainty with which the phase of a carrier may be recovered at areceiver. Thus, broader phase noise spreads result in higher detectionthe ambiguity, which may be quantified as a bit error rate (BER).

Example embodiments of the present invention relate to semiconductorlasers implemented on hybrid Si/III-V platforms having an extremelynarrow quantum linewidth, relative to conventional DFB LDs, of below 20kHz and below 5 kHz and may thus be used in ultra-high speed opticalcommunication networks and other applications that demand a high degreeof coherence, such as interferometric sensing and laser ranging (LIDAR)over long distances.

Moreover, the hybrid platform thus implemented is compatible with theSi-based CMOS (complementary metal oxide semiconductor) process, whichenables on-chip integration with electronics and other optical passivesand thus allows fabrication of complete, small factor devices such asthe SFPx, (small factor pluggable) modules used in many telecomapplications. The compactness and relatively simple structure of thelasers implemented by embodiments of the present invention provides highscalability and allows their deployment in large numbers on onesemiconductor chip, which external cavity and fiber lasers cannotachieve.

Example Coherence Improvements

The hybrid platforms also enable generating narrow linewidths very closeto threshold based on the quality factor of the resonator and thus havelower power consumption relative to conventional DFB LDs in which narrowlinewidths are available typically by increasing pump currents.Notwithstanding high pump currents however, the coherence levelsachievable with conventional (e.g., commercially available) DFB lasersare typically limited to linewidths (Δν)_(ST) in excess of 100 kHz. Thislimitation of conventional DFB LDs appears to be due to high cavitylosses and spontaneous emission, which may be the fundamental mechanismby which coherence is degraded.

Example embodiments of the present invention relate to a high-Qseparated function hybrid laser, which is implemented with an ultra lowloss resonator, deterrence of spontaneous emission and integrationtechniques, which relative to conventional approaches, improve coherence(e.g., linewidth) by over an order of magnitude, and are thus capable ofuse in coherent communications at Tb/s data rates, advanced sensingnetworks, and other useful applications.

Frequency noise is the time derivative of a laser's phase noiseφ_(n)(t). Semiconductor lasers are characterized by a frequency noisespectrum that has two components, one component related to a lowerfrequency component and another component related to a higher frequency.

At low frequency offsets, e.g., below approximately 1 MHz (<˜1 MHz), thefrequency noise has a 1/f-type of spectrum. This 1/f-related noisecomprises largely technical noise of origins not yet well explained. Dueto the low-frequency nature of this technical noise component, itmanifests itself as a relatively slow jitter in the oscillationfrequency of the laser.

At higher frequency offsets (e.g., >1 MHz), the frequency noise ischaracterized by a white noise spectrum of quantum origin, as describedbelow. This quantum white noise spectrum directly translates to aLorentzian field lineshape.

Coherent communication and coherent sensing, LIDAR and relatedapplications typically use measurements over short time scales (<1 μs).The duration of a symbol in high-speed communication network is ˜1 ns.Interferometric delays and measurement times in sensing and LIDARapplications, etc. are <1 μs. These measurements are therefore primarilyimpacted by the (high-frequency) white noise spectrum of thesemiconductor laser frequency noise.

Moreover, the low-frequency 1/f noise can be compensated externallyusing a feedback loop. For example, Pound-Drever-Hall locking to astable cavity, or coherence cloning using an optical phase-locked loopmay be used for external compensation over such low frequency noise.Example embodiments of the present invention relate to addressing thequantum white noise spectrum, and its associated Lorentzian linewidth,which comprise a more fundamental and critical issue.

A fundamental source of phase noise in a semiconductor laser is quantummechanical in origin. The phase noise results from spontaneously emittedphotons in the active region of the laser. With every spontaneousemission event, a photon of random phase is added to the laser field.Under the effect of a large number of spontaneous emission events, thecomplex field of the laser may undergo random walking (e.g., phasediffusion) in the phase plane. FIG. 3 depicts a phasor representation ofan example spontaneous emission event, according to an embodiment of thepresent invention.

For a semiconductor laser, the phase excursion accumulated over ameasurement time interval τ can be most fundamentally expressedaccording to Equation 1, below.

$\begin{matrix}{\left\langle {{{\Delta\phi}(\tau)}}^{2} \right\rangle = \frac{N_{th}R_{sp}}{\left( {2\overset{\_}{n}} \right)\left( {1 + a^{2}} \right)\tau}} & (1)\end{matrix}$In Equation 1, R_(sp) represents the spontaneous emission rate into thelasing mode, N_(th) the carrier density at threshold, n the averagenumber of coherent photons in the same mode, and α the amplitude-phasecoupling coefficient (also known as “Henry factor”). The frequency noiseassociated with this type of quantum phase noise has uniform, or white,spectral distribution. This white noise spectrum results in a Lorentzianfield lineshape, with a Schawlow-Townes spectral linewidth, as shown inEquation 2, below.

$\begin{matrix}{\left( {\Delta\; v} \right)_{ST} = \frac{N_{th}R_{sp}}{\left( {4\pi\;\overset{\_}{n}} \right)\left( {1 + a^{2}} \right)}} & (2)\end{matrix}$

The phasor diagram shown in FIG. 3 represents the effect of onespontaneous emission event on the complex electric field of the laser.The constellation of “dots” depicted in FIG. 3 traces an example randomwalk of the field in the complex plane resulting from a large number ofindependent spontaneous emission events.

Example embodiments of the present invention relate to a semiconductorlaser with significant linewidth reduction over conventional DFB LDs.Lasers are implemented using hybrid Si/III-V integration, a high-Q lasercavity design, spontaneous emission control, and efficient heatmanagement in the hybrid Si/III-V platforms.

To implement these hybrid high-Q lasers with control over spontaneousemission and effective heat management, example embodiments of thepresent invention relate to:

(1) a robust measurement technique to accurately determine the frequencynoise spectrum of semiconductor laser, particularly at very low noiselevels; and

(2) a high-Q separated function Si/III-V laser with high-coherencecharacteristics.

A semiconductor laser prototype is thus implemented with improvedlinewidths, output power of at least 100 μW and side mode suppressionratio (SMSR) of at least 40 dB.

Moreover, a fabrication process is thus implemented that may enablefurther linewidth reduction, e.g., to <5 kHz, as well as wafer-levelscaling. While not an unrelated legacy processes for fabricating ofhybrid, Si/III-V lasers may also be scalable (e.g., to full 2-inchwafers), improved process cost-effectiveness, and throughput may thus beprovided in relation thereto. Example embodiments of the presentinvention use spontaneous emission control and thermal management, anddevelop scalable high-yield fabrication processes and robust measurementtechniques as described herein.

Example Hybrid Silicon/III-V Integration

Heterogeneous integration of III-V semiconductors with silicon allowsthe incorporation of light emission and other active functionalities ona silicon platform. In contrast to legacy integration techniqueshowever, example embodiments of the present invention replace a portionof highly absorbing template III-V material with Si (e.g., using waferbonding), which achieves a very low loss optical platform. TemplateIII-V semiconductors used for conventional laser diodes are about threeorders of magnitude more absorbing than the intrinsic Si used asdescribed and claimed herein. The higher absorbance of III-Vsemiconductors is due primarily to heavy doping in contact and claddingregions.

FIG. 4 depicts a schematic cross-section of a first example hybridSi/III-V Laser, according to an embodiment of the present invention. Inthe hybrid Si/III-V platform thus implemented, the lasing mode isconfigured to reside mostly in the low absorbance Si region. The hybridSi/III-V platform thus allows for the separation and independentoptimization of two significant functions of laser operation, (1) photongeneration, and (2) photon storage. In example embodiments, efficientcarrier injection and photon generation is conducted through the III-Vsemiconductor portion of the hybrid, while the vast majority of thecoherent photons are stored in the low loss silicon portion of thehybrid.

Photon storage may be quantified by a photon cavity lifetime τ_(ph) or,equivalently, the cavity quality factor Q=ω_(o)τ_(ph). Reducing thelosses in the laser cavity has a two-fold effect on the laser's quantumnoise linewidth. First, the cavity loss reduction increases the averagenumber of coherent photons n stored in the cavity at a given injectionlevel. Second, the cavity loss reduces the carrier density N_(th) atthreshold, which reduces the concentration of carriers contributing tospontaneous emission. Given the relationship of n, and N_(th) and cavityloss, or equivalently on the cavity Q, the laser linewidth may be scaledaccording to Equation 3, below.

$\begin{matrix}{\left. \left( {\Delta\; v} \right)_{ST} \right.\sim\frac{1}{Q^{2}}} & (3)\end{matrix}$

An example embodiment is operable for reducing the linewidth bymaximizing the cavity quality factor Q.

The total cavity quality factor Q of a hybrid Si/III-V laser can beexpressed according to Equation 4, below.

$\begin{matrix}{\frac{1}{Q} = {\frac{\Gamma}{Q_{{III}\text{-}V}} + \frac{1 - \Gamma}{Q_{Si}}}} & (4)\end{matrix}$

In Equation 4, Q_(III-V), Q_(Si) represent quality factors associatedwith the optical losses in the III-V region, and in the Si region of thehybrid cavity, respectively. Further, Γ represents the mode confinementfactor in the III-V region. Losses in the III-V region are high;dominated by absorption. Example embodiments of the present inventionincrease the total cavity quality factor Q by implementing devicestructures in which only a very small fraction of the mode resides inthe lossy III-V region.

Example High-Q Cavity Design

The hybrid integration thus implemented improves cavity Q significantlyand reduces linewidth substantially, which improves coherence. As seenwith reference to Equation 4, as more of the light is confined in Siportion of the hybrid structure, losses in Si become increasinglysignificant. For values of Γ<<1, the value of Q_(Si) increases insignificance in relation to maximum achievable total Q. In the siliconportion of the hybrid resonator, the main sources of loss includescattering, radiation, and absorption. Thus, Q_(Si) may be expressedaccording to Equation 5, below.

$\begin{matrix}{\frac{1}{Q_{Si}} = {\frac{1}{Q_{SC}} + \frac{1}{Q_{rad}} + \frac{1}{Q_{abs}}}} & (5)\end{matrix}$

The absorption limited Q for Si-only resonators at telecom wavelengthbands around 1.55 μm is typically Q_(abs)>10⁷.

Example Photonic Resonator

An example embodiment relates to a 1D (one-dimensional) photonic crystalresonator, which is operable for minimizing radiation leakage, settingthe lasing frequency, and ensuring single mode output generation. FIG. 5depicts a top view (not to any scale) of an example high-Q photonicresonator, according to an embodiment of the present invention.

The example resonator may be implemented to comprise two (2) uniformmirror sections of a length L_(m) and a bandgap modulated middle sectionwith a length L_(d), which is operable for localizing a high-Q defectresonance. The insets depict structural characteristics of a unit celland a 3D perspective view of the resonator.

The example photonic resonator thus implemented may attain aQ_(rad)>10⁸, thus eliminating radiation leakage as a critical orsignificant loss-contributing mechanism. Further, optimizing Siprocesses according to an example embodiment reduce scattering due tosurface roughness to Q>10⁶. As the value for Q_(Si)=10⁶ may besignificant in relation to the Si resonator, example embodiments mayreduce linewidth by a factor of 100-1000 (in the limit of Γ→0), inrelation to typical all-III-V InP-based semiconductor lasers for whichQ_(III-V)˜10⁴. As Γ→0 however, consequent reductions in modal gain maycause significant increase in threshold carrier density, which impliesthat an optimal value of Γ for high coherence may exist. Hybrid lasersare also implemented with varying Γ for determining an optimally narrowlinewidth.

IC-Based Control Over Spontaneous Emission

In an example embodiment, the mode confinement factor Γ is tuned withthe incorporation of a low refractive index spacer layer. FIG. 6 depictsa schematic cross-section of a second example hybrid Si/III-V Laser,according to an embodiment of the present invention. An exampleembodiment may be implemented in which the low refractive index spacercomprises a layer of silicon dioxide (SiO2), which is disposed betweenthe Si hybrid portion and the Group III-V hybrid portion. Varying thethickness of the spacer changes the penetration of the evanescent tailof the mode into the III-V portion of the hybrid, which controls thevalue of Γ and thus, the cavity quality factor Q as shown with referenceto Equation 4, above. Further, this tuning affects linewidth as well.

Manipulating the penetration of the evanescent tail in the III-V hybridportion changes the local intensity of the field in the active multiplequantum well (MQW) region, which affects the rate of spontaneousemission into the lasing mode according to Fermi's Golden Rule. Uponanalyzing the spontaneous emission rate R_(sp) into its constituentparameters, the quantum noise linewidth may be expressed according toProportionality 6, below.

$\begin{matrix}{\left( {\Delta\; v} \right)_{ST} \propto \frac{\Gamma_{MQW}{N_{th}^{2}\left( {\Gamma_{{MQW},}Q} \right)}}{Q}} & (6)\end{matrix}$

In Proportionality 6, Γ_(MQW) represents the mode confinement factor inthe MQW region. The presence of Γ_(MQW) in Proportionality 6 accountsfor the modification of the spontaneous emission rate into the lasingmode, due to the change of the mode's intensity in the MQW region. Thethreshold carrier density, represented with N_(th), scales in oppositedirections with Γ_(MQW) and Q. An optimum value of Γ_(MQW) minimizes(Δν)_(ST).

Based on Proportionality 6 and for typical parameter values forInP-based laser diodes, the linewidth reduction (or broadening) factormay be computed as a function of spacer thickness. FIG. 7A and FIG. 7Bdepict representations of quantum noise effects on linewidth for twodifferent values of Q_(Si).

In FIG. 7A, computed quantum noise linewidth reduction (or broadening)factor are plotted as a function of the low refractive index SiO₂ spacerthickness. The linewidth reduction factor comprises a ratio of thelinewidth at a given spacer thickness to the linewidth for a nominalminimum thickness of 50 nm. The areas 71 and 79 correspond to linewidthreduction and broadening, respectively. The solid and dashed linescorrespond to hybrid cavities with Q_(Si)=10⁶ and Q_(Si)=10⁵,respectively. FIG. 7B depicts a plot of the spontaneous emission factoras a function of the thickness of the SiO₂ spacer for two values ofQ_(Si), 10⁵ and 10⁶.

The roll-off and eventual linewidth re-broadening for thicker spacerthicknesses is due to the rise in N_(th) as a result of insufficientgain from the MQW region at extremely low Γ_(MQW) values. A linewidthreduction factor on the order of 100× is computed for values ofQ_(Si)+10⁶. The computed linewidth reduction is significantly downgradedfor Q_(Si)=10⁵, which illustrates the significance of the high-Q Siresonator. An example embodiment is implemented in which such linewidthreduction is achieved with fabrication and measurement of high-Q hybridSi/III-V lasers with spacer thickness that vary between, e.g., 50-200nm, inclusive.

Example Heat Dissipation in the Hybrid Si/III-V Laser Platform

External active cooling is used for precision control over laseroperating temperature to allow its stable and narrow-linewidthoperation. Temperature fluctuations cause the lasing frequency tojitter, which contributes to frequency noise at low frequency offsets.In addition to the ambient temperature, internal heating due to seriesresistances contributes to elevation of junction temperature. Withouteffective heat dissipation, steep temperature gradients may arisebetween the sink and the junction, which may negate the affect of theactive cooling. Elevated junction temperatures then reduce gainsignificantly and enhance degenerative effects from non-radiativecarrier recombination and current leakage, which can increase thresholdcurrent density and reduce slope efficiency and/or output optical power.

Heat dissipation for laser diodes is typically implemented by bondingthe laser die to a thermoelectrically controlled (e.g., Peltier) heatsink. FIG. 8A and FIG. 8B each show typical heat dissipationconfigurations for a laser diode. FIG. 8A depicts a first example heatdissipation configuration, in which the laser is bonded to the heat sinkwith its epitaxial (epi) side configured in an “up” configuration. FIG.8B depicts a first example heat dissipation configuration, in which thelaser is bonded to the heat sink with its epi-side configured in a“down” configuration (opposite to the up configuration).

While epi-side up is a simpler configuration, its longer path (˜100 μm)has a concomitantly higher thermal resistance from the heat source tothe sink, relative to epi-side down. Yet while epi-side down has a moreefficient heat-dissipating path (˜2 μm), relative to epi-side down, ithas a more complex and precise process. Nonetheless, epi-side down diebonding is somewhat more typical for managing heating in high power andquantum cascade lasers and other heat-intensive lasers.

An example embodiment is implemented with epi-side down heat managementapplied over the hybrid Si/III-V platform lasers. While also useful insome implementations, the epi-side up bonding scheme drives heat flowtowards the sink through the silicon handle of its SOI(silicon-on-insulator) wafer. This sink-toward flow situation may beexacerbated first by the presence of a buried oxide layer (BOX), whichhas a thermal conductivity 100× lower than that of silicon. Further, thesink-toward flow may be promoted by the presence of air trenches oneither side of an etched silicon ridge waveguide component. Unique tothe hybrid Si/III-V laser platform, this flow raises the overall thermalresistance of epi-side up bonding schemes.

The hybrid Si/III-V lasers described herein may develop junctiontemperatures significantly higher than conventional III-V LDs, e.g., asseen with “early” thermal roll-offs observed in the L-I characteristicsof hybrid Si/III-V lasers. To promote linewidth reduction, an exampleembodiment is implemented in which epi-side down bonding techniques areused, which is especially significant with modal gain limitations, whichmay arise with use of higher thickness spacers. For example, embodimentfabricates hybrid Si/III-V lasers using epi-side-down bonding, such as aflip-chip mounting. Nonetheless, the hybrid Si/III-V lasers describedherein remain compatible with other hybrid integration processes aswell.

Example Process Flow

An example embodiment of the present invention relates to fabrication ofa hybrid Si/III-V laser. FIG. 9 depicts a flowchart for an examplefabrication process 90, according to an embodiment of the presentinvention. In step 91, electron-beam lithography, followed by plasmaetching, defines active laser features on an SOI wafer substrate. Instep 92, the SOI wafer with the defined laser features is bonded (e.g.,wafer bonding) to the gain medium comprising the Template III-Velements. In step 93, the III-V medium is patterned and a current pathis defined. In step 94, metallic contacts are fabricated so that thelaser device is compatible with flip chip mounting.

An example embodiment may be implemented in which the passive Siresonator components are patterned with e-beam lithography (e.g., VistecEBPG 5000+ lithography, Zeon ZEP 520A nano-photolithographic resist,etc.) and a transfer of resulting patterns to silicon with apseudo-Bosch process (e.g., Oxford 380). The waveguide sidewalls aresmoothened to improve Q_(sc) by growing approximately 15 nm of drythermal oxide (e.g., in which oxidation times are computed using aMassoud-related oxidization model). The oxide is then stripped with HF(hydrofluoric acid; e.g., Transene-Buffer HF-Improved) and approximately20 nm of dry oxide is then re-grown. The silicon chip is then preparedfor direct wafer bonding using acetone and isopropanol (IPA) basedcleaning, followed by an organic strip in a stabilized mixture ofsulfuric acid (H₂SO₄, aka HOSOOOH and hydrogen peroxide (H₂O₂ aka HOOH)such as Nanostrip (e.g., Cyantek) or the like for a 1 min duration orso.

An unpatterned III-V chip with an epi-structure is prepared throughacetone and IPA cleans followed aqueous ammonium hydroxide (NH₄OH:H₂O)in a corresponding 1:15 solution for 10 minutes or so. The chip surfaceson each opposite side are activated for bonding with five (5) or sotreatments in oxygen plasma at 200 W (Suss NP12). The chips are thendirectly bonded. For example, the chips may be bonded with a mutualcompression together, e.g., with forces approximating bringing them intocontact and applying light pressure with tweezers, although the precisebonding mechanism remains unclear as to spontaneous bonding, orpressure-induced bonding, respectively. The bonded chips are thenannealed at 150° C. for 1 h, followed by 285° C. for 5 h. After thebonding and/or annealing, the InP (Indium Phosphide) handle is removedin aqueous hydrochloric acid at e.g., a 1:3 HCl:H₂O concentration.

An example embodiment may be implemented using ion implantation (e.g.,H+, 170 keV, 5×10⁻⁴ cm⁻²) and an AZ5214E mask to define a current pathabove the silicon waveguide. A p-contact to p+ —InGaAs (high-p-typeIndium Gallium Arsenide) is formed over the current path by depositinglayers of titanium, platinum and gold (Ti/Pt/Au, 20 nm/50 nm/150 nm) andlifting off image-reversed AZ5210 resist. A mesa is created in the III-Vmaterial by wet-etching down to the n-contact layer (e.g., using Piranha₂SO₄:H₂O₂:H₂O, 1:1:10, respectively for 7 s and HCl:H₂O, 1:2,respectively, for 17 s, Piranha 45 s). An n-contact to n+ InP(high-n-type Indium Phosphide) by depositing Germanium/Gold/Nickel/Gold(Ge/Au/Ni/Au, 30 nm/50 nm/12 nm/225 nm).

The die is thinned to 150 μm and cleaved into bars. Individual bars areannealed at 325° C. for 30 s. An aluminum oxide (Al₂O₃) anti-reflectioncoating of 250 nm is disposed over each of the opposing, substantiallyparallel facets of the bars.

Example embodiments may be implemented that measure frequency noiseusing Mach-Zehnder interferometry (MZI) for frequency discrimination.The differential delay is kept shorter than the expected laser coherencetime. The corresponding MZI free-spectral range (FSR) comprises 847 MHz.The MZI interferometer converts laser phase fluctuations to intensityfluctuations when biased at quadrature and measured with a high speedphotodetector, the spectrum of which is obtained on an RF spectrumanalyzer. The choice for the interferometer delay may comprise atrade-off between the frequency scan range and the frequency gain.

Accurate results may be obtained in which the interferometer remains atquadrature for the duration of a high-resolution measurement of thefrequency spectrum. Hybrid lasers thus under test (e.g., not beingpackaged), may be particularly sensitive to environmental or ambienttemperature fluctuations, causing the laser center frequency to driftout of quadrature. The MZI may thus be locked in quadrature withnegative electronic feedback to the pump current of the laser. Thisfeedback loop bandwidth is kept below 100 Hz, which suffices forsuppressing low-frequency temperature-induced fluctuations, whileleaving the higher-frequency noise spectrum unaffected.

Example High-O Separated Function Hybrid Si/III-V Laser

FIG. 10 depicts a cross-section of an example High-Q separated functionhybrid laser (SFL) device 1000, according to an embodiment of thepresent invention. Laser 1000 represents the hybrid laser diode depictedin either or both of FIG. 4 or FIG. 6. FIG. 11A depicts a perspective ofthe example High-Q SFL laser 1000, according to an embodiment of thepresent invention. FIG. 11A depicts an example grating component of theHigh-Q SFL device 1000, according to an embodiment of the presentinvention.

Example embodiments of the present invention maximize the phasecoherence of a semiconductor lasers with a spatial separation of two (2)of its major functions: the generation of photons, and the storage ofphotons. Existing DFB LDs, which may be constructed using III-V materialcomponents according to conventional approaches, inevitably compromisebetween such functions and in large part, it is this compromise thatconstrains or degrades the phase coherence performance in conventionalLDs.

Example embodiments of the present invention relate to a two-materialhybrid Si/III-V platform, which circumvents or obviates compromisingbetween photon generation and photon storage and improves their phasecoherence significantly over conventional LDs. In example embodiments,photons are generated largely in a component comprising a III-Voptically active material, and photons are stored in a low-optical lossSi component. The III-V material is thus put to its most efficient andsuitable use—photon generation—free from the conventional constraints ofaccommodating waveguiding and the photon storage functions. The storageand waveguiding are more efficiently in the low-loss Si materialcomponent.

As shown in FIG. 10, photon storage occurs in the low-loss Siliconcomponent and the high-Q hybrid Si/III-V resonator as part of the lasercavity. The high-Q resonator stores a large amount of optical energy fora given power output and thus acts as an optical flywheel, which reducesthe effects of phase perturbations caused by quantum mandatedspontaneously-emitted photons generated in the III-V material.

Moreover, increasing the cavity Q reduces the total population ofexcited electrons and holes required to reach the oscillation threshold,which translates directly and proportionally to a decreased rate ofloss-causing spontaneous emission events. Example embodiments may beimplemented using Si substrates compatible with available wafer-scaleprocessing techniques for fabrication of optoelectronic circuitry,devices and packages.

The effects of the factors limiting the laser coherence are evident inan expression for the phase excursion according to Equation 7, below.

$\begin{matrix}{{\left\langle {{{\Delta\theta}(\tau)}}^{2} \right\rangle = {\frac{R}{2\overset{\_}{n}}\left( {1 + \alpha^{2}} \right)r}},} & (7)\end{matrix}$

In Equation 7, R represents the spontaneous emission rate (s⁻¹) into thelasing mode, n represents the average number of coherent photons in theoptical resonator, a represents the linewidth enhancement factor due tocoupling of amplitude and phase fluctuations, and τ represents thesymbol duration (s).

Reducing the overall optical losses in the laser resonator makes thevalue of R smaller, while for a given power emission, increases thevalue of n. This double benefit of lowering the losses is highlighted byrecasting Equation 7 in the form shown according to Equation 8, below.

$\begin{matrix}{{\left\langle {{{\Delta\theta}(\tau)}}^{2} \right\rangle = {\frac{4\pi^{2}{hv}_{o}^{3}\eta}{Q^{2}P}\left( {1 + \alpha^{2}} \right)r}},} & (8)\end{matrix}$

In Equation 8, ν₀ represents the lasing center frequency, η representsthe population inversion factor, which characterizes the increase in thespontaneous emission rate due to partial inversion, P represents thetotal optical power emitted into the lasing mode, and Q represents thecold cavity, loaded quality factor of the laser resonator.

Enhancing the quality factor Q of the resonator leads to a major (e.g.,proportional to Q²) linewidth reduction. To maximize Q, it is helpful toseparate it, conceptually, into the different optical loss mechanisms ofwhich it is comprised, according to Equation 9, below [18].

$\begin{matrix}{{\frac{1}{Q} = {\frac{1}{Q_{rad}} + \frac{1}{Q_{ba}} + \frac{1}{Q_{sc}} + \frac{1}{Q_{e}}}},} & (9)\end{matrix}$

In Equation 9 the subscripts of the loss components indicate radiation,bulk absorption, scattering, and external loss through the cavity'smirrors, respectively. The loss factors Q_(rad), Q_(ba), and Q_(sc)represent sources of a loss Q_(i) intrinsic to the resonator, which setsan upper bound on the maximum possible Q of the resonator. The resonatorloading, Q_(e), determines the fraction of stored energy that is tappedas useful output through the laser mirrors. There is a trade-off inlaser design between stored energy and useful output.

The high-Q hybrid resonator comprises a silicon waveguide, which ispatterned with a 1D grating, as depicted in FIGS. 11B and/or 12A. The Siwaveguide determines the oscillation frequency and tailors thelongitudinal mode profile of the Laser. The coupling to radiation modes(Q_(rad) ⁻¹) is minimized via a bandgap-modulated “defect” (e.g.,nonuniform) section, disposed in the middle (or nearly so) of anotherwise uniform grating, as depicted in FIG. 12A.

The defect is designed directly in the frequency domain, e.g., byparabolically modulating the lower frequency band edge of the grating asa function of position along the resonator, as depicted in FIG. 12B.This quadratic modulation acts as a potential well, localizing aresonant mode with a Gaussian-like profile in both real and reciprocalspace (FIG. 12D, 12E, respectively), not dissimilar to the ground stateelectron wavefunction in a quantum harmonic oscillator. Selecting a welldepth V, comprising an offset frequency from the uniform grating bandedge, and its spatial width L_(d), example embodiments localize the modetightly in a k-space, which reduces significantly coupling to thecontinuum of radiation modes.

Example embodiments relate to fabricating a device in which thefrequency band edge profile is translated to a grating structuremodulation by varying the transverse diameter W_(y) of etched holesalong the length of the resonator, as shown in FIG. 12A. To minimizescattering loss Q_(sc) ⁻¹, the waveguide is fabricated with a shallowrib geometry, to “bury” the mode in the Si slab as shown in FIG. 10 andthus isolate it from the roughness of the etched sidewalls. Bulkabsorption Q_(ba) ⁻¹, predominantly due to free carrier absorption inheavily-doped III-V material, is reduced by the transverse mode profileminimizing the fraction of the optical energy stored in the III-Vmaterial. For example, with a Si device layer choice of a 500 nm, 75% ofthe modal energy is confined to the low absorption Si component and awayfrom the lossy III-V component.

Uniform grating reflectors of length L_(m) on either side of the defectdetermine the fraction of the total power generated in the active regionthat is coupled as useful output, and therefore the external loading ofthe resonator (Q_(e)). As total loaded Q is maximized to reduce phasenoise, the high-Q hybrid resonators are implemented with significantundercoupling, in which Q_(e)>Q_(i). Using such a loading regime withconventional DFB LDs causes susceptibility to spatial holeburning-induced mode instability and linewidth rebroadening due to theirsharply-peaked spatial mode profile. In example embodiments on the otherhand, such spatial hole burning is mitigated by significantly broaderGaussian longitudinal mode profiles, which allows considerableundercoupling and, therefore, large stored energies in the cavity.

FIGS. 12A and 12B, inclusive, depict features of an example high-Qhybrid resonator, according to an example embodiment of the presentinvention. FIG. 12A depicts a top view schematic of an example geometryof an ultra-low-loss grating disposed in Si. FIG. 12B depicts spatialband structure for an example high-Q hybrid resonator, plotted againstthe simulated transmission spectrum, for design parameters (V,L_(d))=(300 GHz, 100 μm), respectively, wherein V represents the depthof the photonic well with respect to the low frequency mirror band edgeand L _(d) the length of the grating defect section. FIG. 12C depicts adispersion diagram of an example local unit cell. The Eigenfrequenciesf_(v), f_(c) correspond, respectively, to modulated frequencydistribution f_(v)(x), f_(c)(x) of the resonator spatial band structure.FIG. 12D depicts a simulated intensity profile for the longitudinalfield of an example high-Q hybrid resonator. The upper (e.g.,grey-shaded) area denotes the defect. FIG. 12E depicts the Fouriercomponent amplitude distribution of the longitudinal field of an examplehigh-Q hybrid resonator. The grey-shaded (e.g., middle) area denotes thecontinuum of radiation modes. FIG. 12F depicts the simulated emissionspectrum of an example high-Q SFL. The grey-shaded area (e.g., betweenthe 2 higher peaks) denotes the resonator bandgap.

As described herein (e.g., with reference to FIG. 9), an exampleembodiment implements the fabrication of high-Q SFLs. Low-optical lossgratings may be disposed on the Si component using e-beam lithographyand plasma etching, which may be followed by die-scale direct bonding tothe III-V material component. The hybrid Si/III-V resonators may beimplemented based on photonic wells with an example design parameter setof (V, L_(d))=(100 GHz, 200 μm), which produces a single localizeddefect mode. Radiation-limited quality factors on the order of 10⁷ areestimated on the basis of Fourier analysis for the selected designparameter set. The length of the distributed Bragg reflectors on eitherside of the defect is set to significantly undercouple the resonator,with a Q_(e) computed to approximate 5×10⁶, based on the assumption of apredominantly III-V absorption-limited intrinsic Q for the exampleresonator. Example cavity lengths, including the reflectors, are in therange of 1 mm.

As described herein (e.g., with reference to FIG. 9), an exampleembodiment implements the fabrication of high-Q SFLs. Low-optical lossgratings may be disposed on the Si component using e-beam lithographyand plasma etching, which may be followed by die-scale direct bonding tothe III-V material component. The hybrid Si/III-V resonators may beimplemented based on photonic wells with an example design parameter setof (V, L_(d))=(100 GHz, 200 μm), which produces a single localizeddefect mode.

Radiation-limited quality factors on the order of 10⁷ are estimated onthe basis of Fourier analysis for the selected design parameter set. Thelength of the distributed Bragg reflectors on either side of the defectis set to significantly undercouple the resonator, with a Q_(e) computedto approximate 5×10⁶, based on the assumption of a predominantly III-Vabsorption-limited intrinsic Q for the example resonator. Example cavitylengths, including the reflectors, are in the range of 1 mm.

FIG. 13A depicts optical power plotted against pump current over variousoperating temperatures, according to an example embodiment of thepresent invention. FIG. 13B depicts light (optical power output) plottedagainst pump current over various operating temperatures, according toan example embodiment of the present invention. FIG. 13C depicts opticalspectra of example High-Q SFLs at a given pump current and at a certaintemperature, according to an embodiment of the present invention. FIG.13D depicts optical spectra of example High-Q SFLs of various gratingperiods at a given driving current and a certain temperature, accordingto an embodiment of the present invention.

Testing the example high-Q SFLs unpackaged and/or on a temperaturecontrolled stage, example embodiments have been implemented to achievesingle-mode, continuous-wave (CW) laser operation with thresholdcurrents as low as 30 mA and single-side output powers as high as 9 mWat room temperature (20 C), as shown in FIG. 13B). Lasing may occur overtemperatures spanning a range from 10° C. to 75° C., as shown in FIG.13A. Single-mode oscillation is observed to be implemented over awavelengths, which span a range of 45 nm between approximately 1530nm-1575 nm, inclusive, in lasers with appropriately varying or differentgrating periods.

FIG. 13C shows a representative optical spectrum of an example high-QSFL. Side mode suppression ratios exceeding 50 Decibels (dB) areobtained at each of the operating wavelengths, as shown in FIG. 13D).Experimental optical spectrum agree with the spectra simulated for boththe passive and active resonators, as shown in FIGS. 13B and 13F,respectively. The lasing mode appears, as predicted from suchsimulation, near the low frequency band edge (offset˜60 GHz), with thestrongest side mode appearing just outside the low frequency band edge.

Example embodiments characterize the temporal coherence of the high-QSFL with measurement of the spectral density of the frequencyfluctuations, which avoids ambiguities inherent to conventionalself-heterodyne measurement techniques in discriminating between lowfrequency (e.g. 1/f) and high frequency noise contributions to thespectral linewidth. Displaying the noise as a function of frequencyallows resolution or separation of the individual noise mechanisms. Thehigh frequency components of the noise spectrum may be more significantthan the low frequency noise components, because the high frequencyportion of the noise spectrum affects high-data rate opticalcommunications more significantly.

FIG. 14 depicts a frequency noise spectrum of an example high-Q SFL,according to an embodiment of the present invention. Two (2) distinctregions in the plot can be discerned. A first region, which reaches upto approx. 100 kHz, displays a 1/f-type dependence, while a secondsegment has a gentler slope that extends up to 100 MHz. The hybridhigh-Q SFL laser diodes of example embodiments are characterized by thetrend shown in FIG. 14. The observed frequency noise spectrum is largelydominated by noise having a technical origin (e.g., noise caused bylaser driving electronics). A level white noise floor is not clearlyrepresented in these data. Thus, an upper bound on the spontaneousemission-induced phase noise may be expressed in terms of aSchawlow-Townes linewidth by using the value of the spectral density atthe high-frequency end and multiplying it by 2π for the two-sidedspectra measured experimentally in relation to the example embodimentsimplemented according to the present invention. The narrowest linewidththus attained has a valued of 18 kHz, measured at a pump current of4.5×I_(th) (e.g., 160 mA).

In FIG. 14, frequency noise is expressed in relation to power spectraldensity (PSD) in Hz²/Hz as a function of the offset frequency from thecarrier, taken at a pump current of 160 mA (4.5×I_(th)) at 20° C. Thespikes shown at near the 100 kHz and 100 MHz frequencies correspondrespectively to current source electronic noise and FM radio noise. ThePSD value near 100 MHz, multiplied by 2n, yields a Schawlow-Towneslinewidth of 18 kHz. The FIG. 4 inset portion includes the fullfrequency noise spectrum as obtained, e.g., using RF spectrum analysis,which shows the feedback-suppressed low frequency end of the spectrum,as well as the onset of the Mach-Zehnder Interferometric (MZI) roll-offat 200 MHz for a free spectral range (FSR) of 847 MHz.

FIG. 15A depicts a Schawlow-Townes linewidth of an example High-Q SFL asa function of the offset pump current from a threshold, according to anembodiment of the present invention. FIG. 15B depicts a distribution ofSchawlow-Townes linewidths as a function of their respective emissionwavelengths for example High-Q SFL devices implemented over multiplelaser bars fabricated on several separate semiconductor chips, accordingto an embodiment of the present invention.

Deviation from the expected linewidth dependence on pump current isobserved in the form of a linewidth floor. This deviation is due, atleast in part, to increased side mode competition observed in theoptical spectra of the lasers and may be likely caused by spatial holeburning. Narrow-linewidth performance is demonstrated across the entireC-band, as shown in FIG. 15B obtained from lasers with varying gratingperiods, and which span different chips and laser bars.

FIG. 16A depicts a modal energy distribution typical of conventionallasers. Most of the energy is distributed in the active region ofconventional semiconductor lasers. In contrast, example embodiments ofthe present invention distribute most of the energy apart from theactive region.

For example, a laser resonator according to an example embodimentincludes an active material, which amplifies light associated with anoptical gain of the resonator, and passive materials disposed inproximity with the active material. The resonator oscillates over one ormore optical modes, each of which corresponds to a particular spatialenergy distribution and resonant frequency.

FIG. 16B depicts an example modal energy distribution for a laser,according to an embodiment of the present invention. Based on acharacteristic of the passive materials, for the particular spatialenergy distribution corresponding to at least one of the optical modes,a preponderant portion of optical energy is distributed apart from theactive material (e.g., III-V material).

The passive materials may include a low loss material (e.g., Si), whichstores the preponderant optical energy portion distributed apart fromthe active material, and buffer material (e.g., SiO₂) disposed betweenthe low loss (Si) material and the active (III-V) material, whichcontrols a ratio of the optical energy stored in the low loss materialto a portion of the optical energy in the active material.

Example embodiments of the present invention thus relate to a new typeof high-coherence semiconductor laser, which is based on deliberatespatial separation of two (2) of the more significant functions of thelaser: light generation in a photogeneratively efficient III-V materialcomponent and light storage in a low-optical loss Si component. Thesefunctions, together substantially determinative the laser coherence. Theimproved coherence is due, predominantly, to a major increase of the Qfactor of the example laser resonators described herein, which displaylaser linewidths as narrow as 18 kHz.

To characterize the performance of these example embodimentstechnologically, lasers with the degree of phase coherence thus enabledcomfortably enable implementation of 16-QAM (quadrature amplitudemodulation) coherent communication schema, LIDAR and otherhigh-performance applications at significant savings over the electricalpower demands characterizing conventional DFB LDs. Moreover, lasersimplemented as described herein are also amenable to on-chip integrationof optical and electronic functions.

Example embodiments of the present invention are thus described inrelation to a resonator for a laser device. The laser resonator has atleast one active material for amplifying light associated with anoptical gain of the resonator. The laser resonator also has one or morepassive materials disposed in proximity with the at least one activematerial wherein the resonator oscillates over one or more opticalmodes, each of the one or more optical modes corresponding to aparticular spatial energy distribution and resonant frequency, andwherein, based on a characteristic of the one or more passive materials,for the particular spatial energy distribution corresponding to at leastone of the one or more optical modes, a preponderant portion of opticalenergy is distributed apart from the active material.

The one or more passive materials may include a low loss material forstoring optical energy of the preponderant portion distributed apartfrom the active material. The one or more passive materials may alsoinclude a buffer material disposed between the low loss material and theat least one active material for controlling a ratio of the opticalenergy stored in the low loss material to a portion of the opticalenergy in the active material. The buffer material may include amaterial like silicon dioxide, which has a low refractive index. Theactive material may include a III-V material and the low loss materialmay include silicon. The passive materials may be disposed in layers, atleast one of which is bonded (e.g., with wafer bonding) with a layerwith active material.

The low loss passive material may be configured with a pattern of holes.The configured hole pattern may determine an oscillation frequency, anoutput rate and an output mode profile of the resonator and detersspontaneous emission therefrom. The pattern of holes may have aone-dimensional (1D) configuration, such as a linear or near linearaspect. The configured pattern of holes may include an approximatelyuniform array of holes of approximately uniform size, and a defect(e.g., related to the approx. uniform size) disposed within (e.g., over,in or nearly in a center area of) the approximately uniform array ofholes.

Example embodiments of the present invention also relate to laserdevices with such resonators disposed on semiconductor dies. The laserdevices may have a heat sink component attached to the semiconductor dieand configured therewith for removing heat generated in the activematerial from the resonator of the laser device. The heat sink may beattached to the semiconductor die with an epitaxial-side-downconfiguration in relation to the active material of the laser resonator(or in other configurations). The laser device may also have a detectorcomponent attached to the semiconductor die and configured therewith fordetermining an output characteristic of the laser resonator. The outputcharacteristic may relate to measuring a frequency noise relatedcomponent of the resonator output, which may include computing a highfrequency noise spectrum of the resonator output and suppressingmeasurement of low frequency fluctuations of the resonator output.

Example embodiments of the present invention relate to a method forlasing, which includes amplifying light associated with an optical gainin at least one active material of an optical resonator, anddistributing spatial energy within the resonator, in which one or morepassive materials are disposed in proximity with the at least one activematerial, in which the resonator oscillates over one or more opticalmodes, each of the one or more optical modes corresponding to aparticular spatial energy distribution and resonant frequency, and inwhich, based on a characteristic of the one or more passive materials,for the particular spatial energy distribution corresponding to at leastone of the one or more optical modes, a preponderant portion of opticalenergy is distributed apart from the active material.

The one or more passive materials may include a low loss material forstoring optical energy of the preponderant portion distributed apartfrom the active material, as well as a buffer material, which isdisposed between the low loss material and the at least one activematerial, and effectively controls a ratio of the optical energy storedin the low loss material to a portion of the optical energy in theactive material. Example embodiments relate to lasers, e.g., systems,devices products, etc. for sustaining such lasing processes, as well asto methods for fabricating them.

A separated function laser is described herein, including the referenceslisted below. The laser has a substrate layer, a storage and waveguidinglayer adjacent to the substrate layer, and a photon generation layeradjacent to the storage and waveguiding layer. The storage andwaveguiding layer comprises a grating pattern adjacent to the photongeneration layer.

Example embodiments of the present invention are described above inrelation to a resonator for a laser device. The laser resonator has atleast one active material for amplifying light associated with anoptical gain of the resonator. The laser resonator also has one or morepassive materials disposed in proximity with the at least one activematerial wherein the resonator oscillates over one or more opticalmodes, each of the one or more optical modes corresponding to aparticular spatial energy distribution and resonant frequency, andwherein, based on a characteristic of the one or more passive materials,for the particular spatial energy distribution corresponding to at leastone of the one or more optical modes, a preponderant portion of opticalenergy is distributed apart from the active material.

Example embodiments of the present invention relate to a method forlasing, which includes amplifying light associated with an optical gainin at least one active material of an optical resonator, anddistributing spatial energy within the resonator, in which one or morepassive materials are disposed in proximity with the at least one activematerial, in which the resonator oscillates over one or more opticalmodes, each of the one or more optical modes corresponding to aparticular spatial energy distribution and resonant frequency, and inwhich, based on a characteristic of the one or more passive materials,for the particular spatial energy distribution corresponding to at leastone of the one or more optical modes, a preponderant portion of opticalenergy is distributed apart from the active material.

The one or more passive materials may include a low loss material forstoring optical energy of the preponderant portion distributed apartfrom the active material, as well as a buffer material, which isdisposed between the low loss material and the at least one activematerial and effectively controls a ratio of the optical energy storedin the low loss material to a portion of the optical energy in theactive material. Example embodiments relate to lasers, e.g., systems,devices products, etc. for sustaining such lasing processes, as well asto methods for fabricating them.

Thus, a laser resonator is described, which includes an active materialfor amplifying light associated with an optical gain of the resonator,and passive materials disposed in proximity with the active material.The resonator oscillates over one or more optical modes, each of whichcorresponds to a particular spatial energy distribution and resonantfrequency. Based on a characteristic of the passive materials, for theparticular spatial energy distribution corresponding to at least one ofthe optical modes, a preponderant portion of optical energy isdistributed apart from the active material. The passive materials mayinclude a low loss material, which stores the preponderant opticalenergy portion distributed apart from the active material, and a buffermaterial disposed between the low loss material and the active material,which controls a ratio of the optical energy stored in the low lossmaterial to a portion of the optical energy in the active material.

As described above, semiconductor lasers have traditionally followed the“common-sense” approach of maximizing the available gain for the opticalmode by placing the multiple quantum well regions in the center of themode, as shown in FIG. 1 16A. The drawback of this approach is that themode is present exclusively in the III-V semiconductor, which is verylossy, and therefore increases the linewidth by both increasing thecarrier density as well as reducing the number of quanta. The approachof the present disclosure, as shown in FIG. 16B, instead controls therate of spontaneous emission into the lasing mode by reducing theoptical field intensity at the quantum well region, and moving the modeinto the low-loss silicon region. In other words, by reducing the loss,the quality factor of the cavity is improved by orders of magnitude,thereby reducing the linewidth. An important insight is that thereduction in modal gain by reducing the field at the quantum wells iscompensated by the reduction of modal loss, so that the thresholdcurrent and optical power are unaffected by this design.

The reduction of spontaneous emission by transverse mode control offersseveral advantages, especially compared to external-cavity designs wherethe gain medium is coupled to a long passive section in order to reducelinewidth:

-   -   1. The field intensity varies exponentially in the transverse        direction; therefore small changes in the fabrication dimensions        enable orders of magnitude improvement in laser linewidth        without compromising the small device footprint or ease of        integration with other optoelectronic components.    -   2. There are no coupling regions between active and passive        waveguide sections, and therefore no additional losses or need        for precise alignment.    -   3. Transverse mode control for linewidth reduction is        independent of the longitudinal mode control, which makes the        technique more versatile for several laser designs, e.g., single        mode DFB/DBR lasers, tunable lasers, etc.

The technique for spontaneous emission control presented in the presentdisclosure is versatile and can be readily extended to a variety oflaser designs, some of which are listed in the following. Since thewavelength of oscillation of the laser is lithographically determined tohigh accuracy, an array of lasers at the same wavelength (e.g., forphased arrays) or different wavelengths (e.g., forwavelength-division-multiplexed communication systems along with amultiplexer or switch) can be readily fabricated on the same photonicintegrated circuit. The technique is independent of the type of lasercavity, and can be applied to other laser designs as long as the lossesin the resonator are sufficiently low. Some laser designs of particularinterest include: 1. Tunable lasers, e.g., lasers including multipleVernier-tunable grating reflectors to select the wavelength of emission.The gratings etched in the passive medium (silicon) can be controlled bytemperature—using integrated local heaters placed in close proximity tothe grating and/or the tuning sections—or by current injection. Tunablelasers that span the C-band, or more, are of particular interest forhigh-speed telecommunication networks. 2. Resonator designs with high Q,including DFB resonators, DBR resonators, discrete mode lasers etc. Insome embodiments, separate electrical contacts can be added to thedevice, to inject electrical currents to electrically tune the Braggreflectors separately.

Additional optical or optoelectronic feedback and/or filtering can beemployed to further reduce both the white frequency noise as well as the1/f noise at low frequencies. The integrated laser on a hybrid platformenables these techniques to be achieved on-chip. These methodsinclude: 1. Optical injection locking from a stabilized high-Q passiveoptical resonator. 2. Electronic stabilization of the laser and removalof frequency noise using negative feedback. The use of a feedback loopto stabilize the laser to a high-Q optical resonator or filter byvarying its injection current or temperature results in the reduction ofthe laser frequency noise within the loop bandwidth. Such negativefeedback techniques (optical or optoelectronic) are very useful tosuppress the low-frequency 1/f frequency noise in the laser. 3. Thehigh-frequency phase noise spectrum of the laser can be further reducedby locking the laser frequency to the center of an optical filter with ahigh transmission at line-center and a very narrow linewidth (high-Q).The output of the laser after passing it through the filter has areduced noise spectrum at higher frequencies.

Since the overwhelming majority of the optical mode rests in the passivemedium in this design, the thermal properties of the laser are improved.In particular, decreased absorption of the optical power leads to lowerlocal temperature variations across the mode, thereby eliminatingundesirable effects such as self-focusing or filamentation of theoptical mode. In turn, this enables higher optical output powers in asingle mode laser design than what is achievable by conventional III-Vsemiconductor lasers.

While in the present disclosure exemplary lasers were described asfabricated on a hybrid Si/III-V platform, the fundamental principle ofremoving the light from lossy III-V materials into lossless materialscan be extended to other lossless media and resonator designs. Some ofthese materials may possess more desirable properties than silicon. Forinstance, a passive resonator fabricated on a silicon dioxide or asilicon nitride platform can lead to even higher Q than a siliconplatform, particularly at higher optical powers, where silicon cansuffer from considerable nonlinear absorption. These other materials mayalso have a lower temperature sensitivity compared to silicon, leadingto a greater temperature stability than the hybrid Si/III-V lasersdescribed in the present disclosure. Temperature stability may also beachieved by using an athermal resonator design bonded to the activemedium.

The present disclosure describes a narrow linewidth laser devicecomprising a transverse mode control layer to control the distributionof the optical energy (a preponderant portion in a low loss medium andan evanescent portion in the lossy or gain media) and a high-Q resonatorpatterned in the waveguide to reduce losses.

In some embodiments, a low noise widely tunable semiconductor laser cancomprise the transverse mode control layer to control the distributionof the optical energy, and a high-Q resonator patterned in the waveguideto reduce losses, with the resonator comprising a patterned gratingincluding, for example, tunable gratings, typically using the Verniereffect. Vernier gratings are gratings based on the Vernier effect andcan be employed for wide tuning. In the present disclosure, the Verniergratings are specially designed to achieve a high-Q over the entiretuning range of the laser. The Vernier gratings are then combined withthe transverse mode control layer. The Vernier gratings allow tuning ofthe low noise laser.

In some embodiments, the a thickness of the mode control layer isconfigured to transversely distribute the preponderant portion of theoptical energy apart from the active material and maintain an evanescentfield in the active material. Evanescent fields are known to the personof ordinary skill in the art.

FIG. 17 illustrates an extension of the quantum noise control concept toa tunable sampled grating distributed Bragg reflector (SGDBR) laser. Alaser according to other embodiments of the present disclosure isillustrated (1705) and a laser with tunable Bragg reflectors isillustrated (1710). Each SGDBR grating mirror comprises a uniformgrating that is modulated by a periodic nonuniform sampling function(ideally of the form sin z/z), and the front (1715) and back (1710)mirrors have different sampling periods. Thermal heaters can beincorporated in close proximity to each mirror to change thetemperatures of the mirrors (T₁ and T₂) and enable spectral tuning.

The Quantum Noise Controlled Laser (QNCL) paradigm as described in thepresent disclosure is applicable to any tunable resonator design, seeRef. [61], that maintains a high Q across the tuning range. In thepresent disclosure, tunable resonators can utilize sampled gratingdistributed Bragg reflector (SGDBR) mirrors with slightly differentgrating sampling periods. An SGDBR grating consists of a uniform gratingmodulated by a periodic sampling function (or modulating function). Theperiodicity of the sampling function creates multiple orders ofwavevectors that satisfy the Bragg condition, and the spectralreflectivity of each mirror is consequently a series of equally spacedpeaks, with a spacing corresponding to the sampling period. Theresonator can be thermally tuned using integrated heaters located inclose proximity to each SGDBR mirror. The resonant frequency isdetermined by the Vernier effect, i.e., by the spectral overlap of onepeak in the reflectivity spectrum of each grating mirror. Jayaraman etal., see Ref. [62], describe further details of a SGDBR laser design. Inthe present disclosure, a SGDBR implementation is described, comprisinggrating designs that both influence the QNCL design and are in turninfluenced by the spontaneous emission control technique.

The three main parameters of interest in the design of a resonator for awidely tunable, ultra-coherent semiconductor laser are the wavelengthtuning range, the sidemode suppression ratio (SMSR), and thequality-factor of the resonant mode. The design of sampled gratings toachieve wide tuning range and high SMSR is known to the person ofordinary skill in the art, for example as described in Ref. [62]. TheSMSR is influenced by two types of competing modes: adjacent modes ofthe laser cavity, determined by the cavity length; and adjacent gratingmodes, determined by the sampling period of the grating. While thelatter modes can be efficiently suppressed by the SGDBR design, it isdesirable to have the cavity length of the order of the grating lengthor less for efficient suppression of adjacent cavity modes. It istherefore important that the laser linewidth reduction technique shouldnot increase the length of the device excessively. The design paradigmof the present disclosure naturally separates the linewidth reductionand longitudinal mode control functions, and is inherently suited tothese tunable laser designs.

The third key parameter, namely high-Q (low loss) modes over the entiretuning range, has proven elusive in state-of-the-art SGDBR designs owingto the high losses in the active medium as well as high mirror lossesthat vary across the different grating modes. In some embodiments of thepresent disclosure, this problem can be solved by a hybrid Si/III-VSG-DBR laser design with a sampled-grating-based silicon resonator thatmaintains a very high Q across all the grating modes. For example, Q canbe about 10⁶. The high Q implies that the grating reflectivity peakshave to satisfy two requirements: (1) a very high reflectivity(typically >99.5%) and (2) uniformly high reflectivity for all thegrating peaks across the tuning range of the device.

The reflectivity of the nth mode of a sampled grating is given byR(n)=tan h²(κ(n)×L_(unsamp)), where κ(n) is the coupling coefficient forthe nth mode, and L_(unsamp) is the length of the unsampled grating.This peak is located at a wavevector β(n)=π/Λ₀+n·π/Λ_(samp), where Λ₀and Λ_(samp) are the periods of the unsampled grating and the samplingfunction respectively. The coupling coefficients κ(n) are given by theFourier transform of the sampling function, appropriately normalized.The key to realize a uniformly high reflectivity across all gratingmodes is a nonuniform sampling function (such as with the shape sin z/z)that samples the underlying uniform grating, resulting in a constantvalue of κ(n) across the tuning range. Other approaches, such as phaseor frequency modulated gratings, may also be used to achieve uniformspectral reflectivity peaks with higher efficiency, see Ref. [63].

The nonuniform sampling function creates a structural change to thegrating. The uniform grating with a constant period is spatially variedby a nonuniform sampling function. The physical dimensions of thegratings, such as the length, width, depth or all or some of them, canbe changed according to the sampling function. For example, the functioncan determine how the length, width or depth of the gratingperturbations are modified relative to a uniform grating pattern. Forexample, if the grating comprises holes or grooves in the material, thephysical dimensions of the holes or grooves are modified, relative to auniform pattern, according to the nonuniform function. The samplingfunction can also change the period or spacing of the grating. In theembodiments comprising two or more gratings, the sampling functionapplied to each grating can be the same or different. In someembodiments, the gratings can be tuned by varying their respectingtemperatures through integrated heaters. These heaters are disposed inthe proximity of the gratings, in such a way as to be able to controltheir temperature independently from other gratings. The integratedheaters can be, for example, heating pads or resistors.

The present disclosure describes, in some embodiments, a spatiallyvarying sampled grating on the silicon waveguide (varying along thelength of the waveguide) which results in a uniformly high reflectivityacross all the grating peaks across the tuning range of the device. Insome embodiments, the coupling coefficients κ achieved in experimentallydemonstrated DFB lasers is about 100 cm⁻¹, the desired spacing of thereflectivity peaks of the sampled grating is about 4-5 nm, and a cavitylength (not including the mirror sections) can be about 1-2 mm. Withthese parameters, it is possible to calculate that the overall length ofthe SG-DBR laser device in order to obtain a high Q-factor over allgrating modes is less than 5 mm, which is a practical value for acompact, narrow linewidth, integrated, widely tunable laser design. Insome embodiments, the parameters above may be modified outside the citedrange, depending on the application.

In some embodiments, the tunable gratings, such as two tunable gratingswhich are independently tunable, are part of the mode control layerwhich controls the shifting of optical energy from the active opticallayer to the low loss layer. In other embodiments, the gratings can bepart of the low loss material layer. In yet other embodiments, thegratings can be part of the both the mode control layer and the low losslayer.

Definitions that are expressly set forth in each or any claimspecifically or by way of example herein, for terms contained inrelation to features of such claims are intended to govern the meaningof such terms. Thus, no limitation, element, property, feature, orattribute that is not expressly recited in a claim should limit thescope of such claim in any way. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense. Example embodiments of the present invention relate to providingpower to medical and physiological instruments and exchanging signalstherewith.

In the foregoing specification, example embodiments of the inventionhave been described with reference to numerous specific details that mayvary from implementation to implementation. The specification anddrawings are, accordingly, to be regarded in an illustrative rather thana restrictive sense. The sole and exclusive indicator of the scope ofthe invention, and what is intended by the applicants to be the scope ofthe invention, is the literal and equivalent scope of the set of claimsthat issue from this application, in the specific form in which suchclaims issue, including any subsequent correction. The references in thepresent application, shown in the reference list below, are incorporatedherein by reference in their entirety.

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What is claimed is:
 1. A laser resonator comprising: at least one activematerial providing optical gain for amplifying light within the laserresonator; and one or more passive materials disposed in proximity withthe at least one active material, wherein the one or more passivematerials comprise: a low loss material layer configured to storeoptical energy of a preponderant portion distributed apart from theactive material, a mode control layer disposed between the at least oneactive material and the low loss material layer, wherein: the low lossmaterial layer or the mode control layer or both the low loss materiallayer and the mode control layer comprise tunable gratings, the modecontrol layer is configured to control a ratio of the optical energystored in the low loss material to a portion of the optical energy inthe active material, the at least one active material and the one ormore passive materials overlap and extend in a longitudinal directionfor a same length along a laser beam emission direction, the laserresonator device oscillates over a single optical mode in thelongitudinal direction and a single optical mode in a transversaldirection perpendicular to the longitudinal direction, the longitudinalsingle mode and the transversal single mode corresponding to aparticular spatial energy distribution and resonant frequency, based ona characteristic of the one or more passive materials, for theparticular spatial energy distribution corresponding to the longitudinalsingle mode and the transversal single mode, a preponderant portion ofoptical energy is distributed transversally apart from the activematerial, a spectral linewidth for the laser resonator is less than 530kHz, the laser resonator has a quality factor Q greater than 10⁴, and athickness of the mode control layer and the quality factor Q areconfigured to result in a low phase noise for the laser resonator. 2.The laser resonator as described in claim 1, wherein the tunablegratings are Vernier grating.
 3. The laser resonator as described inclaim 1, wherein the tunable gratings comprise a plurality of uniformBragg reflectors modulated with a periodic nonuniform sampling function.4. The laser resonator as described in claim 3, wherein at least one of:length, width, depth, and period of the modulated Bragg reflectors isvaried according to the periodic nonuniform sampling function.
 5. Thelaser resonator as described in claim 4, wherein the plurality ofmodulated Bragg reflectors comprises a first Bragg reflector and asecond Bragg reflector.
 6. The laser resonator as described in claim 5,wherein the periodic nonuniform sampling function is (sin z)/z and thefirst and second Bragg reflectors have different sampling periods. 7.The laser resonator as described in claim 6, wherein the first andsecond Bragg reflectors are configured to increase the quality factor Qof the laser resonator.
 8. The laser resonator as described in claim 7,wherein the first and second Bragg reflectors are tuned by separateelectrical injection currents.
 9. The laser resonator as described inclaim 7, further comprising a first integrated heater disposed inproximity to the first Bragg reflector, and a second integrated heaterdisposed in proximity to the second Bragg reflector, the first andsecond integrated heaters being configured to tune the laser resonatorby changing temperatures of the first and second Bragg reflectors. 10.The laser resonator as described in claim 9, wherein the at least oneactive material is shaped as a mesa structure and the low loss materialextends laterally beyond the mesa structure.
 11. The laser resonator asdescribed in claim 10, wherein the mode control layer comprises silicondioxide.
 12. The laser resonator as described in claim 10, wherein theactive material comprises a III-V material.
 13. The laser resonator asdescribed in claim 12, wherein the low loss material comprises silicon.14. The laser resonator of claim 1, wherein the quality factor Q is atleast 10⁵.
 15. The laser resonator of claim 1, wherein the spectrallinewidth is less than 75 kHz.
 16. The laser resonator of claim 1,wherein the spectral linewidth is less than 18 kHz.
 17. The laserresonator of claim 1, wherein the spectral linewidth is less than 5 kHz.